1. Field of the Invention
The present invention relates to new type porous cordierite ceramics having a new characteristic interconnecting open cellular structure. The present invention relates also to a hollow microspherical cordierite glass powder devoid of any pore or provided with an abundance of open pores in a shell thereof, which is suitable as a starting material for producing the porous cordierite ceramics and to a process for producing same by spray thermal decomposition of a starting solution. The present invention relates further to a use of the porous cordierite ceramics as a carrier for gas chromatography or various chemical reactions, e.g. catalytic or biochemical reactions.
2. Description of the Prior and Related Arts
From the past, porous cordierite ceramics distinguished by their excellent thermal shock resistance and chemicals-resisting property have widely been utilized, usually after being processed to various shaped bodies, as heat insulating materials, filters, catalyst supports and carriers on which enzymes or microbes are immobilized. In this case, porosity of the cordierite ceramics largely contributes to their useful applications. In order to enhance the function of these useful materials, a precise control is required for various factors with respect to porosity of such cordierite ceramics. In fact, many attempts have been made to control the size of pores, pore volume, pore size distribution, shape of pores and pore structure of cordierite ceramics. Up to the present, however, none of the prior art reported hitherto succeeded in controlling all of these factors.
On the other hand, inorganic hollow glass spheres are utilized as light-weight aggregates, carriers for catalysts and heat-insulating materials. Such inorganic hollow glass spheres are generally manufactured by softening a powdery gel as a film-forming material or a naturally occurring glass-forming material such as a volcanic ash under heating and foaming the softened material. In addition to such foaming method under heating, various methods have been proposed for producing hollow spherical substances. Among the methods using a starting material in the form of a solution or slurry, some utilize a volatile liquid vehicle and others utilize chemical reactions resulting from pyrolysis. Concerning the latter-mentioned mode of reaction, a number of methods for producing powdery substances have long been used wherein a solution or slurry of starting materials is subjected to spray thermal decomposition. This "spray thermal decomposition" itself is already known from some 50 years ago and widely used for producing metal oxides from salts, especially in the field of metallurgy. Depending on the conditions, products of spray thermal decomposition are obtained in the form of hollow microspheres. Thus, several prior art techniques are known as a means for producing hollow vitreous microspheres according to spray thermal decomposition. In U.S. Pat. No. 2,797,201, for example, there is disclosed a method for producing hollow spherical shells by preparing a solution of a film-forming substance and a foaming substance in a volatile solvent and atomizing the solution into a spray drying column from a top thereof whereby droplets of the solution were heated countercurrently by a hot blast while descending, to permit evaporation of the solvent and poreless spherical shells are collected at the bottom of the column. According to the method disclosed in U.S. Pat. No. 2,978,339 wherein a solvent is not used as a mixture medium, fine solid particles obtained by mixing a substance capable of producing glass by fusion with a foaming agent is introduced upwardly into a vertical furnace wherein the particles are fused by a high temperature gas to form hollow vitreous microspheres. U.S. Pat. No. 3,030,215 discloses a method of producing hollow spherical glass powders wherein a slurry containing water glass, zinc oxide, boron oxide, a foaming agent, etc. is dried at about 300.degree. C. for 16 hours and ground, and the resultant particles are foamed in an up-draft maintained at about 1100.degree. C. whereby hollow spherical glass powders are blown up and collected at the top of the up-draft. Further, U.S. Pat. No. 4,021,253 discloses a method of producing a similar hollow spherical glass powder wherein a gel containing a silica sol (Ludox), sodium methoxide, boric acid and urea or a gel obtaining by hydrolyzing ethyl silicate, lead acetate, sodium acetate, potassium acetate, calcium chloride and urea is dried, ground and sieved and the resultant particles of a definite size are then foamed under heating. Recently, U.S. Pat. No. 4,257,799 discloses a method of producing hollow spherical glass powders wherein a solution of a silicate glass-forming substance is dropped within a vertical furnace the temperature of which is precisely controlled, whereby droplets of the solution are solidified and foamed under heating to form hollow spherical glass powders. All of the methods disclosed in these references contemplate the production of hollow spherical glass particles by foaming the raw material under heating optionally by the aid of a foaming agent but do not relate to cordierite glass powders. Furthermore, these prior and related arts contemplate the production of hollow spheres with a shell devoid of any pore therein.
From the past, a general method for producing porous cordierite ceramics comprises crushing and grinding cordierite or starting materials such as silica-alumina-magnesia, magnesite-kaolin or talc-alumina-kaolin, subjecting the ground particles to a sieving treatment to collect particles of a definite size, mixing them with a binder, and then molding and sintering the mixture. In this case, porosity of the sintered bodies results from a number of interstices remaining between the sintered particles. Another general method for producing porous cordierite ceramics comprises mixing cordierite or such starting materials with a combustible substance or a foaming agent and then molding and sintering the mixture. In this case, the pore structure results from interstices and pores formed by combustion or pyrolysis of the combustible substance or foaming agent. Inasmuch as physical properties such as mechanical strength, thermal shock resistance, etc. of the porous cordierite ceramics are largely influenced by porosity, pore size, pore volume, pore distribution, shape of pores, etc., controlling of these pore factors is recently demanded for the porous cordierite ceramics especially for use in catalytic reactions. In U.S. Pat. No. 3,950,175 there is disclosed a method for controlling pore size of porous cordierite ceramics wherein kyanite, pyrophilite, quartz and/or fused silica is substituted for talc or clay in the raw material and the substituted raw material is fired at 1340.degree.-1450.degree. C. to yield a cordierite ceramic having at least 20% open pores larger than 10.mu. in diameter. Recently, U.S. Pat. No. 4,374,044 discloses a method for producing a high strength, high macro-pore volume porous cordierite ceramics wherein a usual dry clay raw material containing talc, clay and alumina preferably with an organic binder is subjected together with a colloidal silica diluted with water to disc pelletizing to form green pellets which are then dried to remove free water and fired at 1390.degree.-1410.degree. C. whereby any organic binder contained is burnt out.
According to the conventional methods for producing porous cordierite ceramics, a dry clay mixture which may partially be substituted by a similar ingredients is commonly used as a raw material and is fired at about 1400.degree. C. without using the so-called "spray-thermal decomposition" method, even in the methods recently reported in the above-mentioned U.S. patents. Particularly noteworthy here is that in the production of porous cordierite ceramics, the pore size and/or pore volume may be controlled by the recently developed methods but control of the shape of pores and the pore structure in addition to the above pore characteristics has never been achieved hitherto. As the function of the porous cordierite ceramics as carriers for various purposes is largely influenced by all of the pore characteristics, there is a great demand for developing a method for controlling all of the pore characteristics of hollow microspherical cordierite glass powders and porous cordierite ceramics.